In the formation of a coating film by applying a urethane paint, an epoxy paint, a polyester (alkyd) paint, an unsaturated polyester (alkyd) paint, a melamine paint, a silicone paint, a siloxane paint, an acrylic paint, or a mixed paint thereof to an inorganic substrate, such as a substrate formed of metal, metal oxide, glass, ceramic, cement, or mortar, the coating film may have poor adhesion to the inorganic substrate.
As a method for preventing this adhesion failure, a method in which a primer layer composed of a silane coupling agent is formed between an inorganic substrate and a coating film formed from the paint described above has been proposed (see, for example, Patent Document 1). Actually, this sometimes improves the adhesion between an inorganic substrate and a coating film.
However, for example, in the formation of a hard acrylic resin film by the application and polymerization of a paint containing a large amount of acrylic compound, such as a polyfunctional acrylic monomer or acrylic oligomer, that can achieve a high cross-linking density as used in hard coating, the film tends to have a relatively large polymerization shrinkage (resulting in a large internal stress and easily causing the detachment of the film) and poor flexibility (resulting in poor followability to temperature change and deformation of a substrate and easily causing the detachment of the film). Thus, a previously proposed method (for example, Patent Document 1) may achieve insufficient adhesion to an inorganic substrate.
Other propositions include a primer composition that contains a certain silane compound and an organic compound having a mercapto group to improve the adhesion to an inorganic material (Patent Documents 2 and 4), a composition suitable for paints that contains a certain silane compound and an organic mercaptan (Patent Document 3), a silicone grease composition that contains a thiol as an anticorrosive (Patent Document 5), and a silane compound having two mercapto groups suitable for semiconductor sealants (Patent Document 6).
However, these compositions and compounds disclosed in Patent Documents 2 to 6 have the following problems.
In the primer composition disclosed in Patent Document 2, the amount of usage of polythiol is smaller than the amount of usage of silane compound (for example, the mercapto group/Si group ratio=0.2 in Example 1). Thus, there is still room for improvement in adhesion between a substrate and a coating film (confirmed by the present application).
The composition disclosed in Patent Document 3 contains a silicon resin having a phenyl group and an aliphatic unsaturated group reactive with a mercapto group and an organic mercaptan having at least two mercapto groups, wherein the mercapto group/unsaturated group ratio (molar ratio) ranges from 0.7 to 2.0. However, when the composition was used as a paint, the composition had such low adhesion that a coating film on a glass plate could be mechanically detached. Thus, there is still room for improvement in adhesion (Example 2). Furthermore, use of the paint as a primer to bond an acrylic cross-linking resin to an inorganic material is not described or even suggested.
A dental adhesive composition disclosed in Patent Document 4 contains a compound that has a mercapto group having a high affinity for organic materials and metals and an alkoxysilyl or alkylsilyl group conducive to adhesion to glass. However, when the composition was used as a primer to bond an acrylic cross-linking resin to a glass plate, the composition exhibited insufficient adhesion between a substrate and a coating film even at 3-SH:3-MPS (weight ratio)=1:1 (mercapto group/(meth)acryloyl group=1.2) at which the adhesive strength was considered to be highest (confirmed by the present application).
In Patent Document 5, as it is reported that a monothiol having 24 or more carbon atoms and a dithiol having more than 16 carbon atoms have low adsorption force for metals, it has been suggested that the addition of a mercapto compound causes deterioration in adhesion.
Although Patent Document 6 proposes a silane compound having two mercapto groups to improve adhesion, raw materials for such a silane compound are difficult to obtain, and water cannot be used in manufacturing. It is therefore difficult to industrially obtain the proposed silane compound or manufacture the proposed silane compound at low cost. There is room for improvement in cost.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-81853
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 7-26207
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 5-156163
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2-152915
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 4-164995
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 4-74190